The present invention relates to ceramic fiber composites, and more particularly to composites with coated ceramic fibers in a ceramic matrix.
Ceramic fiber composites have been attracting increasing interest primarily because of their high toughness levels (e.g., approximately 20 MPam.sup.1/2) and frequent absence of catastrophic failure upon reaching their maximum load carrying capability. The high toughness of these composites has been characterically associated with limited bonding between the fibers and the matrix in order to give substantial fiber pull-out as a major source of toughening. On the other hand, brittle failure in unsuccessful composites and resultant flat fractures with essentially no fiber pull-out have been attributed to strong fiber-matrix bonding.
Good toughness characteristics can be introduced into composites with ceramic fibers and a ceramic matrix only when there is very limited bonding between the fibers and matrix. However, many possible combinations of such ceramic fiber composites result in strong bonding over most, if not all, of the range of practical processing conditions, thereby limiting the potential toughness ranges of these composites. Furthermore, many ceramic fibers will react chemically with a variety of possible ceramic matrixes, destroying or limiting both strength and toughness.
There are other important motivations for investigating coatings for fibers besides controlling fiber-matrix bonding. Fiber coatings can protect fibers from mechanical damage during handling and processing. They can also provide chemical protection from attack by the matrix during the processing. Coatings may also be of importance in limiting the oxidation embrittlement of ceramic fiber composites.
Coatings may be applied to the fibers to limit bonding and chemical reaction with the matrix if the coating has suitable coherence to the fibers. Prior art ceramic fiber composites with metal coated fibers, such as disclosed in U.S. Pat. No. 3,869,335, are suitable for low temperature processing and use environments, but not at elevated temperatures because of the relatively low melting point of the metal coatings.